[llvm] [InstCombine]Fold Phi Zext's to a singular Zext (PR #214045)

[Andreas Jonson via llvm-commits]

https://github.com/andjo403 commented:

to get you started with some comments:
- the Alive proof is not for this PR
- lots of comments in the code that have strange formatting and most are redundant/confusing 
- more test is needed

https://github.com/llvm/llvm-project/pull/214045